Example sentence: but the ecstasy in which he lay drifting seemed that of a man no longer of this world
Here is a definition of "this world" that seems to apply:
Merriam-Webster "this world"
the world known to living men : the world of here and now
The dictionary contrasts that with "otherworld"
: a world beyond death or beyond present reality
So, "no longer of this world" then means that the individual has lost connection to the here and now, to present reality.